Touch sensing or touch position detection technology capable of providing a natural interface between an electronic system and user has found widespread applications in a variety of fields, for example, in mobile phones, personal digital assistants (PDAs), automatic teller machines (ATMs), game machines, medical devices, liquid crystal display (LCD) devices, light emitting diode (LED) devices, plasma display panel (PDP) devices, computing devices, and the like, where a user may input desired information and/or operate the electronic system through a touch sensing device associated with the electronic system. A touch sensing device typically includes a controller, a sensing circuit having a plurality of touch sensors and a network of control lines electrically connecting the plurality of touch sensors to the controller, and a touch panel associated with the plurality of touch sensors.
Recently, in-cell touch sensing panels in which an LCD incorporates touch sensors have been developed. Those touch sensors sense the pressure, capacitance or ambient light brightness difference caused by a touch of a finger or a stylus and provide electrical signals corresponding thereto for the LCD.
FIG. 16 shows a circuit diagram of a conventional in-cell touch sensor including a reset thin-film transistor (TFT), RST-TFT, an amplification TFT, AMP-TFT, a selection (read) TFT, SEL-TFT, a storage capacitor, Cref, and a liquid crystal capacitor, Clc. For such a touch sensor, a touch detection includes three steps: initialization, charging and readout.
Initialization: when the voltage level of Gate line G1 turns high, RST-TFT is turned on. Accordingly, the voltage level at the node VA is initially set to Vinit.
Charging: when the voltage level of Gate line G1 turns low, RST-TFT is turned off. The voltage level at the node VA is shifted to a certain level if a touch exists,
            V      A        =                  V        init            -                                                  C              ref                                                      C                ref                            +                              C                lc                            +                              C                p                                              ·          Δ                ⁢                                  ⁢        V              ,where Cref is a reference capacity, Clc is a liquid crystal capacity of the pixel, which is variable with a touch event, Cp is a parasitic capacity of the pixel, and ΔV is the difference between the high voltage level and the low voltage level of the Gate line G1.
Readout: when the voltage level of Gate line G2 turns high, SEL-TFT is turned on, thereby reading out the signal (from a sense line).
FIG. 17 shows a conventional photo-type touch sensor includes a source follower TFT, M1, a reset TFT, M2, a column bias TFT, M3, a photo sensor, PD and a storage capacitor, CINT. When M2 is turned on, the storage capacitor CINT is charged to a certain voltage level, and the photo sensor PD leaks according to the intensity of its ambient light. As a result, Vint is reduced to a different level after a period of time. When RWS is pulsed on, the level of Vint is increased, because of the capacitive coupling. Further, the voltage level of Vpix is also increased from the level of VSS to a certain level, for example, a power supply voltage VDD, thereby determining that a touch event occurs. FIG. 18 shows timing charts of operation signals of the photo-type touch sensor as disclosed above.
For the touch sensor designs, the sensed touch signal amplification is utilized with a Mamp TFT, for example, AMP-TFT in FIG. 16, or M1 in FIG. 17. Sometimes, sensed touch signals may be very weak, so that even after the signal amplification, one may still be unable to determine whether a touch event exists. Thus, the sensitivity of the touch sensor designs is limited.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.